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The Signal Line Circuit (SLC) carries the power and data for addressable alarm causing devices on an addressable panel (Notifier 9600, for example). The Indicating Device Circuit (IDC) carries the power and signal for conventional (shorting) alarm causing devices on a conventional panel (Silent Knight 5208, for example).

The Signal Line Circuit (SLC) carries the power and data for addressable alarm causing devices on an addressable panel (Notifier 9600, for example). The Indicating Device Circuit (IDC) carries the power and signal for conventional (shorting) alarm causing devices on a conventional panel (Silent Knight 5208, for example).

FYI, these are terms used in NFPA 72. Proprietary systems may use a different term, but that just makes for confusion. Particularly, Simplex (now Johnson Controls) calls their old system MapNet and the new system IDNet. Both are SLC's

They are the same in that the are connected to the same types of devices (initiating). They work differently though. Conventional fire alarm systems use IDC circuits/zones, mainly now used for sprinkler monitoring systems. You can put one type of device on each IDC/zone. For example, only smoke detectors on one zone, pull stations on another, water flow's on the next, etc. It works like on/off or open/closed circuit. SLC has power and data in the one circuit and are used in much larger systems. You can mix smoke detectors, pull stations, etc on this same circuit. Each device has it's own address as well to make it easier to identify.